JP4499966B2 - Secondary battery charging circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a charging circuit for a rechargeable secondary battery, and more particularly, to a charging circuit for a secondary battery capable of rapid charging and preventing noise in a frequency band that affects devices used such as a mobile phone. Is.
[0002]
[Prior art]
As charging methods for lithium ion batteries, a constant current-constant voltage charging method and a pulse charging method are often used. In the constant current-constant voltage charging method, the charging time is shortened by increasing the charging current for the lithium ion battery or by making the constant voltage applied to the lithium ion battery slightly higher than the full charge voltage of the battery during charging. Can do. However, if the lithium ion battery is overcharged, the performance of the battery may be reduced. On the other hand, the pulse charging method is a method in which the battery is less damaged because the charging pause period for the lithium ion battery is taken.
[0003]
In such a pulse charging method, there are the following three methods.
As a first method, as disclosed in Japanese Patent Application Laid-Open No. 6-113474, the charging is completed when the voltage during the idle period reaches a predetermined voltage.
As a second method, the charging is stopped when the first voltage is reached during charging, and the charging is restarted when the voltage is lowered to the second voltage. Charging is repeated, and charging is completed when the charging stop period is equal to or longer than a predetermined time, or when the ratio of the charging period to the charging stop period exceeds a predetermined value.
As a third method, as disclosed in Japanese Patent Application Laid-Open No. 7-336908, charging is alternately repeated at a high level and a low level voltage, and the charging current at the low level becomes a predetermined current value or less. When charging is complete.
[0004]
[Problems to be solved by the invention]
However, the first method has a problem that the charging time is longer than that in the constant current-constant voltage charging method. In the second method, the charging time is slightly shortened as compared with the constant current-constant voltage method, but each time of the charging period and the charging stop period varies greatly between the start of charging and the end of charging. For this reason, the repetition frequency of the charging period and the charging stop period changes in a wide range, and thus there is a problem that noise occurs over a wide frequency band.
[0005]
Further, in the third method, a current detection means for detecting a charging current at a low level is required. Since the current detection element is inserted in series in the charging circuit as the current detection means, power loss is reduced. In addition, since it is necessary to increase the value of the current detection resistor in order to detect that the charging current has become zero, the power loss is further increased and a complicated circuit is required. There was a problem.
[0006]
The present invention has been made in order to solve the above-described problems. With a simple circuit, the charging time can be shortened, and the occurrence of noise in the frequency band that affects the equipment used can be prevented. An object of the present invention is to obtain a rechargeable battery charging circuit.
[0007]
[Means for Solving the Problems]
A charging circuit for a secondary battery according to the present invention is a charging circuit for a secondary battery that charges a secondary battery such as a lithium ion battery, and has a plurality of constant voltages set in advance according to an input control signal. Selecting and outputting one, applying a constant voltage to the secondary battery for charging, a battery voltage detecting circuit for detecting and outputting the battery voltage of the secondary battery, A control circuit unit that performs switching control of a constant voltage output from the constant voltage circuit unit according to a detection voltage from the battery voltage detection circuit unit, and the control circuit unit controls the constant voltage circuit unit. When the battery voltage of the secondary battery is equal to or lower than a predetermined first constant voltage, charging is performed by applying the first constant voltage to the secondary battery, and the battery voltage of the secondary battery is equal to the first constant voltage. If it exceeds, a predetermined second constant voltage and a predetermined third constant smaller than the second constant voltage. Is intended to perform the charging alternately it is applied to the secondary battery pressure at a constant period.
[0008]
In addition, when the control circuit unit causes the constant voltage circuit unit to apply the third constant voltage to the secondary battery, if the battery voltage of the secondary battery exceeds a predetermined charge completion voltage, It was detected that the charging of the battery was completed, and a predetermined charging completion operation was performed.
[0009]
Specifically, the second constant voltage is set to the same voltage as the first constant voltage.
[0010]
Further, the second constant voltage may be larger than the first constant voltage.
[0011]
On the other hand, a load circuit unit for connecting a load to the secondary battery in parallel according to the constant voltage output from the constant voltage circuit unit is provided, and the load circuit unit outputs a third constant voltage from the constant voltage circuit unit. During this time, a load was connected in parallel to the secondary battery.
[0012]
Specifically, the constant voltage circuit unit generates and outputs the first constant voltage, the second constant voltage, and the third constant voltage, and a control signal from the control circuit unit. A voltage switching circuit that selects and outputs one of the first constant voltage, the second constant voltage, and the third constant voltage output from the constant voltage generation circuit, and a constant output from the voltage switching circuit. Voltage comparison between the voltage and the battery voltage of the secondary battery, and a voltage comparator that outputs a signal according to the comparison result; and a current according to the comparison result from the voltage comparator from a predetermined DC power source A control transistor that outputs to the secondary battery and a diode that blocks a current flowing from the secondary battery to the DC power supply via the control transistor are provided.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Next, the present invention will be described in detail based on the embodiments shown in the drawings.
FIG. 1 is a diagram showing a configuration example of a charging circuit for a secondary battery in the first embodiment of the present invention. In FIG. 1, a charging circuit for a lithium ion battery used for a mobile phone is shown as an example.
In FIG. 1, a charging circuit 1 for a secondary battery includes an adapter detection circuit 2 that outputs a predetermined signal when a power supply voltage from an AC adapter 10 that forms a DC power supply exceeds a predetermined value, and a lithium ion battery that forms a secondary battery. 11 includes a battery voltage detection circuit 3 that detects and outputs a positive voltage 11 (hereinafter referred to as a battery voltage) Vb, and a constant voltage circuit 4 that charges the lithium ion battery 11 with a constant voltage.
[0014]
Further, the charging circuit 1 includes a precharge constant current circuit 5 that precharges the lithium ion battery 11 with a predetermined constant current, a signal from the adapter detection circuit 2, and a detection voltage from the battery voltage detection circuit 3. A charge control circuit 6 for causing the constant voltage circuit 4 to charge the lithium ion battery 11 in a pulse charge system and for the precharge constant current circuit 5 to perform the precharge, and the lithium ion battery 11 are connected in parallel. And a load circuit 7.
[0015]
The constant voltage circuit 4 generates and outputs three predetermined constant voltages E1 to E3, respectively, and the constant voltage generation circuit 21 in response to a control signal from the charge control circuit 6. A voltage switching circuit 22 that selects any one of the constant voltages E1 to E3 and outputs it as a reference voltage Vr, an operational amplifier 23 that forms a voltage comparator, and a charging current from the AC adapter 10 to the lithium ion battery 11 The control transistor 24 is a PMOS transistor that controls the supply of the signal, the diode 25, and the gate control circuit 26 that controls the operation of the control transistor 24 according to the output signal from the operational amplifier 23. The charging control circuit 6 is a control circuit unit, in which the constant voltage E1 is a first constant voltage, the constant voltage E2 is a second constant voltage, and the constant voltage E3 is a third constant voltage.
[0016]
Between the power supply terminal 31 to which power is supplied from the AC adapter 10 and the ground, the control transistor 24, the diode 25, and the lithium ion battery 11 are connected in series so that the charging current is supplied to the lithium ion battery 11. It is connected to the. The diode 25 is for preventing a current from flowing backward from the lithium ion battery 11 to the AC adapter 10 when the voltage at the power supply terminal 31 is smaller than the battery voltage Vb of the lithium ion battery 11.
[0017]
The voltage switching circuit 22 selects any one of the constant voltages E1 to E3 according to the voltage switching signal Ss from the charging control circuit 6 and outputs the selected voltage to the inverting input terminal of the operational amplifier 23. The battery voltage Vb of the lithium ion battery 11 is applied to the non-inverting input terminal of the operational amplifier 23, and the output terminal of the operational amplifier 23 is connected to the gate of the control transistor 24 via the gate control circuit 26. The operational amplifier 23 is driven and controlled by a control signal from the charge control circuit 6.
[0018]
On the other hand, the load circuit 7 is formed by a series circuit of a resistor 35 and an NMOS transistor 36, and the resistor 35 and the NMOS transistor 36 are connected in series between the positive electrode of the lithium ion battery 11 and the ground. . The NMOS transistor 36 operates according to the constant voltage selected by the voltage switching circuit 22 and turns on so that the resistor 35 becomes a load for the control transistor 24 of the constant voltage circuit 4. The constant voltages E1 to E3 have a relationship of E2 ≧ E1> E3. When the voltage switching circuit 22 selects the constant voltage E3 as the reference voltage Vr by the voltage switching signal Ss, the NMOS transistor 36 is turned on, and the constant voltage E1 or When E2 is selected as the reference voltage Vr, the NMOS transistor 36 is turned off and is turned off.
[0019]
In such a configuration, FIG. 2 is a timing chart showing an operation example of the charging circuit 1 shown in FIG. 1, and an operation example of each part of FIG. 1 will be described with reference to FIG.
First, the charging control circuit 6 is activated when power is supplied from the AC adapter 10 and a predetermined signal is input from the adapter detection circuit 2. The battery voltage detection circuit 3 detects the battery voltage Vb of the lithium ion battery 11 and outputs the detected voltage value to the charge control circuit 6.
[0020]
When the battery voltage Vb of the lithium ion battery 11 is equal to or less than the predetermined value V1, the charge control circuit 6 activates the precharge constant current circuit 5 to precharge the lithium ion battery 11 with the predetermined precharge current Ip. Let it begin. At this time, the charging control circuit 6 stops the operation of the operational amplifier 23 so that no current flows to the lithium ion battery 11 via the control transistor 24.
[0021]
The predetermined value V1 may be set to about 2.5V in the case of a 4.2V lithium ion battery 11, for example. This is because when the lithium ion battery 11 is suddenly charged with a large current when the lithium ion battery 11 is in an overdischarged state, a problem occurs. Therefore, at the start of charging, the charging current is reduced to precharge the lithium ion battery 11. Do. The precharge current Ip is a current for this purpose and is usually set to a current value of several mA to several tens mA.
[0022]
When the battery voltage Vb of the lithium ion battery 11 rises to a predetermined value V1, the charge control circuit 6 determines that the lithium ion battery 11 is a normal battery and terminates the precharge by the precharge constant current circuit 5 The voltage switching signal Ss is output to switch to constant voltage charging by the constant voltage circuit 4. During the precharge period, the operation of the constant voltage circuit 4 is stopped, and the diode 25 prevents current from flowing from the lithium ion battery 11 to the AC adapter 10 at this time.
[0023]
When the precharge is completed, the charging control circuit 6 causes the voltage switching circuit 22 to select the constant voltage E1 by the voltage switching signal Ss, and the selected constant voltage E1 is output to the inverting input terminal of the operational amplifier 23 as the reference voltage Vr. Is done. In the constant voltage circuit 4, the output voltage becomes the constant voltage E1, and the lithium ion battery 11 is charged with the constant voltage E1. The charging current Ic when charging the lithium ion battery 11 at the constant voltage E1 is as shown in FIG. 2, and a constant current limited by the current capacity of the AC adapter 10 or the current capacity of the control transistor 24 is The charging current Ic is output from the constant voltage circuit 4.
[0024]
When the battery voltage Vb of the lithium ion battery 11 gradually increases and reaches a voltage E1 equal to the output voltage of the constant voltage circuit 4, the charge control circuit 6 pulse-charges the lithium ion battery 11 to the constant voltage circuit 4. Operation control is performed so that the battery is charged by the method. In addition, in the case of a lithium ion battery, the constant voltage E1 is good to set to 4.2V which is a full charge voltage.
[0025]
The pulse charging method is a method of charging the lithium ion battery 11 by repeatedly switching the output voltage of the constant voltage circuit 4 between the constant voltage E2 and the constant voltage E3 at a predetermined cycle. When the voltage of the lithium ion battery 11 reaches the voltage E1, the charging control circuit 6 outputs a voltage switching signal Ss to the voltage switching circuit 22 so as to select the constant voltage E3, and the output voltage of the constant voltage circuit 4 is Set to constant voltage E3. The constant voltage E3 is lower than the constant voltage E1, but is set to such a voltage that a sufficient charging current Ic can be output to the lithium ion battery 11 immediately after switching to the pulse charging method. For example, the constant voltage E3 may be set to be 4.0V to 4.1V in the case of a 4.2V lithium ion battery.
[0026]
Next, the charging control circuit 6 outputs a constant voltage E3 to the voltage switching circuit 22 after a predetermined time T1 has elapsed since the voltage switching signal Ss is output so as to select the constant voltage E3. The voltage switching signal Ss is output so as to select E2. The voltage switching circuit 22 selects and outputs the constant voltage E2 so that the output voltage of the constant voltage circuit 4 becomes the constant voltage E2. The constant voltage E2 may be set to the same voltage as the constant voltage E1, or may be set to be slightly larger than the constant voltage E1, for example, about 0.1V. FIG. 2 shows an example where the constant voltage E2 is larger than the constant voltage E1.
[0027]
When the constant voltage E2 is set to the same voltage as the constant voltage E1, an overvoltage is not applied to the lithium ion battery 11, so that there is no fear of damaging the lithium ion battery 11, and a voltage common to the constant voltage E1. Therefore, the circuit can be simplified, but there is a disadvantage that the charging time is slightly longer. When the constant voltage E2 is set slightly higher than the constant voltage E1, the charging time can be shortened and the possibility of damaging the lithium ion battery can be reduced because of the pulse charging method.
[0028]
Next, the charging control circuit 6 outputs the voltage switching signal Ss so as to select the constant voltage E2 to the voltage switching circuit 22 and then again determines the voltage switching circuit 22 after the predetermined time T1 has elapsed. The voltage switching signal Ss is output so as to select the voltage E3. The voltage switching circuit 22 selects and outputs the constant voltage E3 again so that the output voltage of the constant voltage circuit 4 becomes the constant voltage E3. In this way, the charging control circuit 6 causes the constant voltage circuit 4 to alternately output the constant voltages E2 and E3 at a constant period until the charging of the lithium ion battery 11 is completed.
[0029]
Here, as can be seen from FIG. 2, immediately after switching to the pulse charging method, the charging current Ic is equal to the AC adapter 10 regardless of whether the output voltage of the constant voltage circuit 4 is the constant voltage E3 or the constant voltage E2. Current capacity or current limited by the current capacity of the control transistor 24 is almost constant. However, as the charging of the lithium ion battery 11 proceeds, the charging current Ic during charging at the constant voltage E3 gradually decreases. Further, when the charging of the lithium ion battery 11 proceeds and the battery voltage Vb of the lithium ion battery 11 becomes equal to or higher than the constant voltage E3, the charging current Ic does not flow when charging is performed at the constant voltage E3. Such a state is a charging method similar to a general pulse charging method in which charging and stopping of charging are repeated, and is a charging method that can prevent damage to the lithium ion battery 11 and extend its life.
[0030]
When the charging of the lithium ion battery 11 further proceeds and the battery voltage Vb of the lithium ion battery 11 when charging at the constant voltage E3 exceeds a predetermined charging completion voltage Ve, the charging control circuit 6 11 is completed, the operation of the operational amplifier 23 is stopped, the operation of the constant voltage circuit 4 is stopped, and the charging operation for the lithium ion battery 11 is stopped.
[0031]
Here, the NMOS transistor 36 of the load circuit 7 is turned on when the voltage switching circuit 22 selects the constant voltage E3. When the NMOS transistor 36 is turned on, the resistor 35 becomes a load of the constant voltage circuit 4. Thus, when the output voltage of the constant voltage circuit 4 is switched from the constant voltage E2 to the constant voltage E3, it is possible to shorten the time for the battery voltage Vb of the lithium ion battery 11 to reach a stable voltage. The time required for the comparison with the charge completion voltage Ve performed by the charge control circuit 6 can be shortened. For this reason, the time for charging the lithium ion battery 11 with the constant voltage E3 can be set short, and the degree of freedom for setting the charging cycle of pulse charging to a frequency that does not affect the equipment used can be increased.
[0032]
FIG. 3 is a flowchart showing an example of the operation of the charge control circuit 6. The operation flow of the charge control circuit 6 will be described with reference to FIG. Unless otherwise specified, the processing performed in each flow is performed by the charging control circuit 6.
In FIG. 3, first, it is detected from the signal input from the adapter detection circuit 2 whether or not the voltage at the power supply terminal 31 has become a predetermined voltage or higher (step S1). If this could not be detected (NO), it continues to detect whether or not the voltage at the power supply terminal 31 has become a predetermined voltage or higher. If it has been detected that the voltage has been higher than the predetermined voltage (YES), It is checked whether or not the battery voltage Vb of the lithium ion battery 11 detected by the battery voltage detection circuit 3 exceeds a predetermined value V1 (step S2).
[0033]
If the battery voltage Vb of the lithium ion battery 11 is equal to or lower than the predetermined value V1 in step S2 (NO), the precharge constant current circuit 5 is operated to precharge the lithium ion battery 11 (step S3). Return to step S2. On the other hand, when the battery voltage Vb of the lithium ion battery 11 exceeds the predetermined value V1 in step S2 (YES), the operational amplifier 23 is operated and the voltage switching circuit 22 is selected to select the constant voltage E1, The lithium ion battery 11 is charged at a constant voltage with a constant voltage E1 (step S4).
[0034]
Thereafter, it is checked whether or not the battery voltage Vb of the lithium ion battery 11 exceeds the constant voltage E1 (step S5). If the battery voltage Vb of the lithium ion battery 11 is equal to or lower than the constant voltage E1 (NO), the lithium voltage continues. It is examined whether or not the battery voltage Vb of the ion battery 11 exceeds the constant voltage E1. On the other hand, when the voltage of the lithium ion battery 11 exceeds the constant voltage E1 in step S5 (YES), the voltage switching circuit 22 is selected to select the constant voltage E3 and The battery 11 is charged with the constant voltage E3 (step S6).
[0035]
Next, it is checked whether or not a predetermined time T1 has elapsed since the start of charging at the constant voltage E3 (step S7). If the predetermined time T1 has not elapsed (NO), the predetermined time T1 is reached. The battery is charged at a constant voltage E3 until elapses. If the predetermined time T1 has elapsed in step S7 (YES), it is checked whether or not the battery voltage Vb is equal to or higher than the predetermined charge completion voltage Ve (step S8), and is equal to or higher than the charge completion voltage Ve. In (YES), the charging of the lithium ion battery 11 is completed, and this flow ends.
[0036]
If the battery voltage Vb is less than the charge completion voltage Ve in step S8 (NO), the voltage switching circuit 22 is made to select the constant voltage E2, and the constant voltage circuit 4 causes the lithium ion battery 11 to be connected to the constant voltage E2. (Step S9). Next, it is checked whether or not a predetermined time T1 has elapsed since the start of charging at the constant voltage E2 (step S10). If the predetermined time T1 has not elapsed (NO), the predetermined time T1 is reached. The battery is charged at a constant voltage E2 until elapses. If the predetermined time T1 has elapsed in step S10 (YES), the process returns to step S6.
[0037]
As described above, the charging circuit according to the first embodiment uses the precharge current Ip from the precharge constant current circuit 5 when the battery voltage Vb is equal to or lower than the predetermined value V1 with respect to the lithium ion battery 11. When precharging is performed and the battery voltage Vb exceeds a predetermined value V1, constant voltage charging is performed with the constant voltage E1 from the constant voltage circuit 4, and when the battery voltage Vb reaches the constant voltage E1, constant voltage switching with respect to the voltage switching circuit 22 is performed. The control is performed so that the constant voltages E2 and E3 are alternately output from the constant voltage circuit 4 at a constant cycle, and pulse charging is performed. Therefore, by adding a simple circuit, when charging a lithium ion battery, it is possible to shorten the charging time and to prevent the generation of noise in the frequency band that affects the device used. .
[0038]
【The invention's effect】
As apparent from the above description, according to the charging circuit of the present invention, when the battery voltage of the secondary battery is equal to or lower than the predetermined first constant voltage, charging is performed by applying the first constant voltage to the secondary battery. When the battery voltage of the secondary battery exceeds the first constant voltage, a predetermined second constant voltage and a predetermined third constant voltage smaller than the second constant voltage are alternately secondary in a constant cycle. It was made to charge by applying to a battery. Therefore, constant voltage charging is performed before pulse charging, so that charging can be performed with a large current. After pulse charging starts, charging is performed by switching between two constant voltages of high level and low level at a constant cycle. Therefore, the charging current can be continued and the charging time can be shortened, and the switching cycle can be set to a frequency that does not adversely affect the equipment used.
[0039]
In addition, when the third constant voltage is applied to the secondary battery, when the battery voltage of the secondary battery exceeds a predetermined charging completion voltage, it is detected that the charging of the secondary battery is completed, and the predetermined charging is performed. Added completion action. For this reason, overcharge can be reliably prevented.
[0040]
Specifically, the second constant voltage is set to the same voltage as the first constant voltage. Thus, the circuit can be simplified and charging can be performed without damaging the secondary battery.
[0041]
Further, the second constant voltage may be larger than the first constant voltage, and in this way, the high level voltage during pulse charging is made slightly higher than the full charge voltage, and the secondary battery The charging time can be shortened without damaging the battery.
[0042]
On the other hand, while the third constant voltage is output from the constant voltage circuit unit, a load is connected in parallel to the secondary battery. This makes it possible to quickly stabilize the battery voltage of the secondary battery that is charged at the third constant voltage during pulse charging, thereby reducing the detection error of the charging completion voltage and the period of pulse charging. Therefore, the period can be set to a frequency that does not adversely affect the equipment used.
[0043]
Specifically, the constant voltage circuit unit generates and outputs the first constant voltage, the second constant voltage, and the third constant voltage, and a control signal from the control circuit unit. A voltage switching circuit that selects and outputs one of the first constant voltage, the second constant voltage, and the third constant voltage, and a voltage comparison between the selected constant voltage and the battery voltage of the secondary battery Comparator, a control transistor for outputting a current according to the comparison result from the voltage comparator to the secondary battery, and a current flowing from the secondary battery to the DC power supply through the control transistor is blocked. And a diode to be provided. Therefore, the secondary battery can be charged by switching from constant voltage charging to pulse charging with a simple circuit configuration.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration example of a charging circuit for a secondary battery according to a first embodiment of the present invention.
FIG. 2 is a timing chart showing an operation example of the charging circuit 1 in FIG.
3 is a flowchart showing an operation example of the charge control circuit 6 of FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Charging circuit 2 Adapter detection circuit 3 Battery voltage detection circuit 4 Constant voltage circuit 5 Precharge constant current circuit 6 Charge control circuit 7 Load circuit 10 AC adapter 11 Lithium ion battery 21 Constant voltage generation circuit 22 Voltage switching circuit 23 Operational amplifier 24 Control transistor 25 Diode 26 Gate control circuit

Claims (6)

In a secondary battery charging circuit that charges a secondary battery such as a lithium ion battery,
A constant voltage circuit unit that selects and outputs one of a plurality of preset constant voltages according to the input control signal, and applies the constant voltage to the secondary battery for charging.
A battery voltage detection circuit for detecting and outputting the battery voltage of the secondary battery;
A control circuit unit that performs switching control of a constant voltage output from the constant voltage circuit unit according to a detection voltage from the battery voltage detection circuit unit;
With
The control circuit unit causes the constant voltage circuit unit to charge the secondary battery by applying the first constant voltage when the battery voltage of the secondary battery is equal to or lower than a predetermined first constant voltage, When the battery voltage of the secondary battery exceeds the first constant voltage, a predetermined second constant voltage and a predetermined third constant voltage smaller than the second constant voltage are alternately applied to the secondary battery at a constant cycle. A charging circuit for a secondary battery, characterized in that charging is performed. The control circuit unit applies the third constant voltage to the secondary battery with respect to the constant voltage circuit unit, and if the battery voltage of the secondary battery exceeds a predetermined charging completion voltage, The charging circuit for a secondary battery according to claim 1, wherein the charging is detected, and a predetermined charging completion operation is performed. The secondary battery charging circuit according to claim 1, wherein the second constant voltage is the same voltage as the first constant voltage. The secondary battery charging circuit according to claim 1, wherein the second constant voltage is larger than the first constant voltage. A load circuit unit connected in parallel to the secondary battery according to the constant voltage output from the constant voltage circuit unit, the load circuit unit outputs a third constant voltage from the constant voltage circuit unit; 5. The secondary battery charging circuit according to claim 1, wherein a load is connected in parallel to the secondary battery. The constant voltage circuit unit is:
A constant voltage generation circuit for generating and outputting the first constant voltage, the second constant voltage, and the third constant voltage, respectively;
A voltage switching circuit that selects and outputs one of the first constant voltage, the second constant voltage, and the third constant voltage output from the constant voltage generation circuit in response to a control signal from the control circuit unit; ,
A voltage comparator that performs a voltage comparison between the constant voltage output from the voltage switching circuit and the battery voltage of the secondary battery, and outputs a signal according to the comparison result;
A control transistor for outputting a current corresponding to a comparison result from the voltage comparator from a predetermined DC power source to the secondary battery;
A diode that blocks a current flowing from the secondary battery to the DC power source via the control transistor;
A charging circuit for a secondary battery according to claim 1, 2, 3, 4 or 5.

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